TORQUE STEER

Copyright AA1Car

Torque steer is the annoying tendency of a front-wheel drive car to pull to one side during hard acceleration. The problem they say, is caused by an "imbalance" in the distribution of power to the front wheels. So what does that mean, and what can you do about it?

It means torque steer is an inherent though undesirable characteristic of front-wheel drive. Its causes are complex but it can be cured.

CAUSE & EFFECT OF TORQUE STEER

The transaxle in a typical front-wheel drive car with a transverse mounted engine is offset to one side to physically accommodate the engine within the engine compartment. This means the position of the differential gears is also off-center, which requires unequal length driveshafts. Unequal length driveshafts, however, have been found to deliver more torque to the wheel with the shorter driveshaft (which is usually the left wheel). Under light loads, the difference is insignificant. But under hard acceleration, the wheel with the shorter shaft gets more torque, causing that wheel to pull harder than the other wheel. The stronger wheel tends to pull ahead of the other wheel, which creates the induced steering pull towards the opposite side. Thus the direction of torque steer in a vehicle with a left mounted transaxle is usually towards the right.

Under normal driving conditions and moderate acceleration, torque steer is rarely noticeable, unless a vehicle also has unequally inflated tires or an alignment problem, or there is excessive compliance (flex) in the control arm bushings.

Somewhere along the way, engineers discovered that switching to equal length driveshafts helped even out the torque loads, which in turn reduced the tendency to torque steer. The factory "fix" for an offset transaxle, therefore, has been to divide the longer shaft into two parts. A short intermediate shaft is plugged into the long side of the transaxle, with a support bearing at the outboard end to hold it steady. A shortened right side driveshaft, which is equal in length to the left side driveshaft, is then connected to the intermediate shaft to complete the driveline. In theory, the intermediate shaft becomes an extension of the transaxle itself because the shaft is rigidly mounted and does not change angles with motions of the suspension. In practice, it helps to reduce torque steer but it doesn't always eliminate it entirely.

The trick to eliminating torque steer is balancing engine torque as even as possible side-to-side. With equal length driveshafts, a car may pull either right or left during hard acceleration depending on other factors that affect driveline geometry and traction. The slant of the engine in the chassis, for example, can have an affect. That's why some vehicles have shims under the right motor mount. By raising or lowering the right side of the engine slightly, any tendency to torque steer one way or the other can be neutralized. Likewise, changing the relative height of the intermediate shaft bearing by repositioning its mount can be used to fine tune the driveline. Installing stiffer suspension bushings that have less give also helps keep the wheel pointed straight ahead.

CHANGES IN ALIGNMENT GEOMETRY

The thing that causes torque steer is the fact that the front wheels, which do the steering, are now also doing the driving. This completely changes the thrust geometry and how it acts upon the chassis.

In a rear-wheel drive chassis, the driving force generated at the rear axle pushes through the center of the car along an imaginary line called the thrust line. As long as both wheels have equal traction and the rear axle is in alignment with the rest of the chassis (perpendicular to the vehicle centerline), the drive forces are directed straight ahead. If the front wheels are also aimed straight ahead, the car goes straight. But if one or both wheels break traction and start to spin, the rear of the car can "fishtail" or drift sideways. Even so, the driver can still maintain control by countersteering to compensate for the skid.

With front wheel drive, the dynamics are entirely different. It's like riding a tricycle compared to riding a bicycle. You can't ride a tricycle no handed but you can a bike. Every crank of the pedals on a trike induces a steering thrust on the wheel. Not so with a bicycle.

With FWD, the thrust line now points wherever the front wheels point. So like a team of horses pulling a wagon, the wagon goes which ever way you aim the horses. If one or both front tires break traction, they can jerk the wheel and cause the driver to lose steering control. It's also next to impossible to maintain much steering control over wheels that are slipping. The driver can't countersteer because the wheels have already lost traction. The only way to regain control is to back off the throttle and reestablish traction. That's one of the negative aspects of front-wheel drive nobody says much about.

The problem of maintaining steering control can be further aggravated by the fact that the front wheels toe-out when steered. Toe-out is a necessary function built into all steering systems to compensate for the fact that the inner and outer wheels follow different arcs of travel when turning. But it works against front-wheel drive during hard acceleration because it encourages torque steer.

Engine torque also loads the control arm bushings and causes the front wheels to change toe alignment under hard acceleration. The wheel on the

side with the shortest axle (if axles are unqeual in length) will often change toe more than the one with the longer shaft, causing the vehicle to pull

towards the right. When the front wheels change tow and are no longer parallel, the steering will pull toward the stronger side. Worse yet, if the road is wet or slippery, the wheel that is most lightly loaded will be the one that steals all the torque through the differential. This tug of war between the front wheels can be aggravated by increasing tire width and/or wheel offset. If one wheel gains the upper hand, it will break free and spin. Wheel spin isn't a problem with your typical under-powered FWD passenger car, but it can be a problem with higher powered or turbocharged engines in FWD cars.

Traction is further diminished by the fact that physics is also working against front-wheel drive. The rearward weight shift that normally occurs during hard acceleration takes weight off the front wheels and shifts it to the rear -- just the opposite of what's needed to maintain traction.

An obvious cure for the traction problem would be to use a limited slip differential in the transaxle. Yet except for a few high end FWD cars, limited slip differentials are not available as a factory option on most FWD cars. The reason why limited slip differentials have not been offered to date is because engineers have intentionally held horsepower ratings within "safe" limits. Safe, in this case, meaning both what the transaxle and typical driver are capable of handling. The current limit is somewhere around 175 to 200 horsepower.

A locked differential can't be used with FWD because of the adverse effects it would have on steering control. But a "soft" limited slip such as the silicone clutch pack Ford uses in the European Escort or a worm gear arrangement (like a Torsen differential in a Neon SRT) are the best choice for transaxle applications.

DIAGNOSE TORQUE STEER

Chrysler issued on of the first technical service bulletins on the subject way back in 1981 (bulletin #17-03-80). The bulletin applies to 1978-80 Omni and Horizons, but the basic principles apply to all FWD cars. Here's what it said:

"A slight amount of vehicle lead to the right on heavy acceleration from a standing start, most noticeable with manual transmission and manual steering, is the result of a torque steer condition. Some torque steer is normal on front-wheel drive vehicles."

To diagnose the condition, the technician was advised to test drive the vehicle on a level road to rule out road crown as a cause of the steering pull. If the vehicle then showed a tendency to drift or pull to one side while driving at normal speeds, a tire or alignment problem would be the cause. If the vehicle drove normally, however, but still pulled to the right only during acceleration then it was suffering from torque steer.

Chrysler's suggested cure, in this instance, was to check the front suspension height by measuring from the center of the lower control arm pivot bolt to the floor. If the ride height was greater than seven inches, then the cure was to lower the front suspension by replacing the front springs.

Lowering the front end, in this case, changed wheel geometry enough to lessen the effects of torque steer. The same trick can work for many other front-wheel drive vehicles.

In their later turbocharged front-wheel drive cars such as the Daytona and Laser, Chrysler used equal length driveshafts to control torque steer. An earlier Chrysler FWD car with unequal length driveshafts could be retrofitted with a late model transaxle or equal length driveshafts to cure a torque steer problem, but it would be an expensive retrofit.

CONDITIONS THAT CAN INCREASE TORQUE STEER

Any or all of the following conditions can aggravate an existing torque steer problem as well as cause it to appear in a FWD vehicle that previously didn't experience torque steer:

Unequal front tire inflation pressure side to side

Unequal camber (the car will pull towards the side with the greatest amount of positive camber).

Toe misalignment (both static toe or toe-out when steering).

Unequal caster side-to-side

Incorrect ride height (nose too high).

Misalignment between the transaxle, driveshafts and chassis.

Front tires with unequal wear.

Loose or worn wheel bearings

Looseness or worn parts in the steering linkage or rack.

Worn or loose control arm bushings

Control arm bushings that are too soft and allow too much movement under load

Excessive front wheel offset (deep dish aftermarket wheels)

Rear axle misalignment

A dragging brake caliper on one side

Increased engine torque (hopping up the engine in a FWD car that already has a torque steer problem will make torque steer worse).

ELIMINATING TORQUE STEER

The first order of business when a torque steer problem is encountered is to check front tire pressures for equal inflation, and to check for steering play. A test drive on a level road will tell you if there is a continuous steering pull or any steering wander, either one of which would require a thorough inspection of the suspension inspection and an alignment check.

Four wheel alignment is highly recommended because misalignment of the rear axle will prevent the rear wheels from tracking properly. This, in turn, may cause the driver to countersteer slightly (an off center steering wheel) which causes the front wheels to toe out -- and you know the rest.

When performing an alignment check, be absolutely certain you check such things as ride height (front-to-rear and side-to-side), the condition of the wheel bearings (both front and rear), the tires (look for unequal wear or mismatched tires), the condition of the lower control arm bushings (anything that increases suspension compliance can contribute to torque steer), the condition of the rack mounts and tie rod ends. And double-check tire inflation pressures just to be safe. The front wheels should be adjusted as closely as possible to the factory preferred settings with equal camber and caster side-to-side.

If everything checks out okay and a vehicle still exhibits a torque steer during hard acceleration, then you have the following options:

* If it has equal length driveshafts, try shimming the right engine mount up or down and/or repositioning the intermediate shaft bearing to tune out the torque imbalance.

* Experiment with tire pressures. A few pounds difference one way or the other may be enough to cancel out the pull.

* Traditional modifications that stiffen up the suspension (installing stiffer springs, shocks, urethane control arm bushings and lower profile tires) may actually aggravate the condition by making the chassis more responsive, unless you are extremely careful about wheel alignment.